What compute device does the material preview use to render?
By using GPU-Z to see gpu load its easy to see that material preview does not use gpu (0% usage). Usage of CPU on the other hand rises when preview updates.

The problem is also how the material preview is coded.


it refreshes with re-scale instead of just computing more samples or only downsizing the preview.
it refreshes not smartly but on every "change". When some value is clicked but not changed, when new node is added but not connected. When you draw line from output and then cancel etc. this could go on and on.


Lot of things blender already has could use more attention from devs..